1. Field of the Invention
This invention relates to the process for compounding additives and thermoplastic resins.
2. Prior Art
The desirability of introducing additives into thermoplastic resins to improve processability is well known. Nucleating agents to increase the rate of crystallization of thermoplastic resins, in particular, are also well known.
Examples of prior art methods for incorporating an additive into a thermoplastic resin are: (1) dry blending two powders either with or without wetting agents in either a Henschel Mill, a Waring Blendor or a ball mill, wherein one of the powders comprises a thermoplastic resin and the other comprises an additive, then mlet mixing to form a molten mass by means of either a Banbury Mill or on a two-roll mill, and recovering that mass; (2) the method of (1) except using mineral oil as a wetting agent and a three-roll paint mill to form a paste in place of a Henschel Mill, a Waring Blendor, or a ball mill; and (3) using a mortar and pestle with and without wetting agents to grind an additive into a very fine powder followed by extrusion compounding that powder with a second powder on a Killian compounding extruder.
Examples of United States patents directed to incorporating an additive or a nucleating agent into a polyolefin or other thermoplastic resin are U.S. Pat. Nos. 3,207,735 (1965), 3,207,736 (1965), 3,207,737 (1965), 3,207,738 (1965), and 3,207,739 (1965), 3,268,499 (1966), and 4,045,403 (1977). Of particular interest is 3,207,739 where sodium salts of aliphatic and aromatic carboxylic acids are used in crystallizable polypropylene. One disclosed method for incorporating an additive into a resin is to add said additive to the washed polymerization slurry followed by dring of the slurry to form a dry mixture of the additive and polymer. Other disclosed methods involve adding the additive to either a molten resin or a dry resin which is either in the form of a powder or a pellet. In U.S. Pat. No. 3,207,735, the same methods as in U.S. Pat. No. 3,207,739 are disclosed but in addition, in one of the examples, there is set out the use of a solution of an additive in acetone or other suitable volatile solvent which is mixed into a polypropylene powder. The solvent is then thoroughly removed by evaporation under drying conditions and the resulting material also in the form of a powder further homogenized by rolling in a bottle.
Still another method for dispersing an additive into a thermoplastic resin is disclosed in the following United States Pat. No. 3,299,029 (1967) 3,326,880 (1967), 3,327,020 (1967) and 3,327,021 (1967). The method disclosed involved first mixing an aqueous solution of the additive into a crystalline polymer and then precipitating the additive with the aid of a salting-out agent. The precipitated additive presumably coats out on the polymer. The water and the salting-out agent are subsequently removed by mechanical means, for instance by filtration or centrifuging.
U.S. Pat. No. 3,275,715 (1966) discloses the need to use high boiling organic liquids which swell powdered polyethylene or powdered propylene as the solvent for an additive which is to be incorporated into said polyethylene or said polypropylene. The patent states however, that the use of low boiling polar-type solvents such as acetone, methyl-ethyl ketone, methanol, ethanol, etc. to dissolve a desired additive to form a solution thereof results in an inadequate dispersion in the following process: tumble blending said solution and a polyolefin, which may be in the form of a powder, followed by flashing off of the solvent.
British Pat. No. 1,182,001 (published Feb. 25, 1970) avoids the difficulties associated with preparing dispersions with metal salts of carboxylic and sulphonic acids by employing crystallization promoter combinations which themselves crystallize at temperatures below the melting point of the polyolefins in which they are effectively incorporated and which combinations preferably are compatible with the molten polyolefin at least to the extent of the concentrations being employed.
U.S. Pat. No. 3,637,634 (1972) discloses a variation on the use of saltingout agents to produce a fine precipitate of sodium benzoate from a water solution thereof. The patent states that under carefully controlled conditions the following process thoroughly disperses sodium benzoate in polypropylene: dissolving sodium benzoate in water, combining the sodium benzoate solution with excess alcohol or other non-solvents for sodium benzoate to form a gel-like slurry, admixing polypropylene powder or particles and said slurry with agitation to ensure thorough mixing, separating the solids from the resulting polypropylene-containing slurry by filtration, centrifugation, or the like, drying the filter cake at an elevated temperature, and recovering polypropylene powder or particles containing finely divided sodium benzoate.
All of these methods have significant drawbacks when seeking to disperse more than 0.05% by weight of sodium benzoate within polypropylene. One drawback is that often one or more discontinuities in the form of specks are observed in injection molded plaques made from such polypropylene. The specks result from the tendency of sodium benzoate to agglomerate during extrusion. It is to be noted that generally more than 0.05% by weight, as based upon the total composition of sodium benzoate is required in order to have a commercially useful effect on the crystallization rate of polypropylene. Another drawback is that volatile organic solvents as salting-out agents are required. Organic solvents are generally environmentally undesirable and often health hazards.
More detail as to the visual evaluation of sodium benzoate dispersions in polypropylene will be discussed in the examples.
Accordingly, a method for dispersing high melting, water soluble additives within a thermoplastic resin is needed. A high melting material is one that has a higher softening or melting point than the thermoplastic resin into which it is to be dispersed.